In extracorporeal blood treatment, blood is taken out of a patient, treated and then reintroduced into the patient by means of an extracorporeal blood flow circuit. Generally, the blood is circulated through the circuit by one or more pumping devices. The circuit is connected to the patient via a blood vessel access, typically in the form of one or more access devices, such as needles or catheters, which are inserted into a blood vessel of the patient. Such extracorporeal blood treatments include hemodialysis, hemodiafiltration, hemofiltration, plasmapheresis, etc.
In extracorporeal blood treatment, it is vital to minimize the risk for malfunctions in the extracorporeal blood flow circuit, since these may lead to a potentially life-threatening condition of the patient. Serious conditions may arise if the blood vessel access is disrupted, e.g. by an access device for blood extraction (e.g. an arterial needle/catheter) coming loose from the blood vessel, causing air to be sucked into the circuit, or by an access device for blood reintroduction (e.g. a venous needle/catheter) coming loose, causing the patient to be drained of blood within minutes. Other malfunctions may be caused by the blood vessel access becoming blocked or obstructed, e.g. by the access device being positioned too close to the walls of the blood vessel, or by tubing in the extracorporeal blood flow circuit being jammed or kinked.
To this end, an apparatus for extracorporeal blood treatment may include one or more surveillance devices that monitor the integrity of the blood flow circuit and issue an alarm and/or cause appropriate action to be taken whenever a potentially dangerous situation is detected. Such surveillance devices may operate on measurement signals from one or more pressure sensors in the circuit. Conventionally, the monitoring is carried out by comparing one or more measured pressure levels with one or more threshold values. For example, failure in the blood extraction may involve air being introduced into the circuit, whereby the measured pressure may approach atmospheric pressure, or the blood flow being blocked or obstructed, whereby the measured pressure may drop to a low level. A failure in the reintroduction of blood may be detectable as a decrease in the measured pressure. However, it may be difficult to set appropriate threshold values, since the pressure in the circuit may vary between treatments, and also during a treatment, e.g. as a result of the patient moving.
To increase the monitoring precision, WO 97/10013 proposes detecting, as one of several options, a heart signal in the measured pressure and using the heart signal as an indicator of the integrity of the circuit, in particular the integrity of the blood vessel access. The heart signal represents a pressure wave which is produced by the patient's heart and transmitted from the patient's circulatory system to the extracorporeal blood flow circuit via the blood vessel access. Malfunctions in the blood vessel access will disturb the transmission of the heart-generated pressure wave to the circuit, causing the heart signal to change or even disappear. The measured pressure also includes a strong pressure wave produced by the blood pump in the extracorporeal blood circuit. In WO 97/10013, the monitoring involves filtering a measured pressure signal to remove the frequency components that originate from the blood pump, and then detecting the heart signal by analysing the filtered pressure signal. However, if the heart signal is very weak and/or the heart beat frequency lies close to any of the frequency components of the blood pump, the heart signal may become undetectable and an erroneous alarm signal will be generated. Such a situation is not unlikely to occur since the patient's heart beat frequency, and often also the frequency of the blood pump, will vary during a treatment.
Corresponding needs to monitor the integrity of a flow circuit may arise in other fields of technology.